STRUCTURE AND DEVELOPMENTAL RATE 239 



veloped liver and pancreas. I have repeatedly succeeded in per- 

 forming the first experiment with Fundulus embryos by early 

 arrests. The second experiment is more difficult and is not yet 

 completely perfected, though among a large number of cases 

 certain specimens arise in the experiment with underdeveloped 

 livers, but normal mouth and branchial regions. 



The experiments with the alimentary organs are more difficult 

 than those on the eyes and brain, since the former are more diffi- 

 cult to observe and are not all so decidedly expressed in the young 

 embryo. The study of the liver and pancreas must also be 

 largely limited to examination of microscopic sections, while the 

 mouth and branchial arrangements and the eyes and brain are 

 very readily examined in total specimens after some experience. 



The experiments on the nervous and alimentary systems as 

 they now stand make very probable the correctness of the follow- 

 ing proposition. The organs arise as a series of buds which bear 

 a relationship to one another very similar to that existing among 

 the buds of a growing plant. A given bud is dominant or has an 

 'advantage of position' for a limited time during which its rate of 

 oxidation and cell proliferation is higher than that of other poten- 

 tial buds in the system. It grows at this moment and continues 

 to dominate the situation until it has exhausted the advantage, 

 when its proliferation rate decreases and another region attains 

 the advantage and begins to bud to form another organ. If 

 the entire embryo be depressed or has its developmental rate 

 reduced at a moment when a certain bud is proliferating at its 

 height, this bud is more decidedly reduced in its rate than any 

 other portion of the embryo. On resuming a more rapid rate the 

 other slow-going parts are able again to attain their ordinary rates, 

 but the bud in question is unable to regain its extraordinarilj'- 

 high rate and therefore loses its exceptional advantage. This 

 bud may be subsequently unable to express itself, since other 

 parts now arrive at the stage of advantage. 



The problem is then to locate the given critical moments for 

 the several developing organs. By depressing development dur- 

 ing a period which would cover a definite moment one might be 

 able to suppress any given organ at will. With sufficiently re- 



